2176
Anal. Chem. 1989, 61 2176-2178 ~
Voltammetric Reduction and Determination of Hydrogen Peroxide at an Electrode Modified with a Film Containing Palladium and Iridium James A. Cox* and Robert K. Jaworski Department of Chemistry, Miami University, Oxford, Ohio 45056
Cyclic voltammetry of a mixture contalnlng 0.2 mM Na,IrCi,, 0.1 mM PdCi,, 0.2 M K,S04, and 0.1 M HCI between 1.2 and -0.3 V vs Ag/AgCI for flve cycles at 50 mV s-' yields a stable fHm on a glassy carbon electrode. The reductlon of hydrogen peroxlde In 0.1 M KCI is diffusion controlled at that modHled electrode. Calibration curves obtalned at a 100 mV 8-' scan rate are hear In the range 0.2-1.8 mM H20,. The slope, 28 pA L mmoi-', Is independent of film thlckness. Since dlssolved oxygen Is reduced at about the same potential as H,O,, -0.3 V, at the modlfled electrode, it will act as an interferent In solutions that are not deaerated; however, the currents are additlve. A second limitation of the described procedure Is that with the KCi electrolyte the l"0Mlked flhn must be reoxidired prior to each measurement. Preliminary data are described whlch suggest that this problem is alieviated by switching to a bask supporting electrolyte.
INTRODUCTION The electrochemical determination of hydrogen peroxide often is used in dip-type sensors and amperometric detectors for biomolecules which yield this species upon enzymatic oxidation. Present systems monitor the hydrogen peroxide by its oxidation at Pt, which was initially reported as a quantitative method by Harrar ( I ) . Because the oxidation is performed a t a quite positive potential, ca. 0.9 V vs SCE, severe interference can occur unless selectivity is achieved with an attendant separation method or through a chemical step. Oxidation by linear scan voltammetry at a glassy carbon electrode avoids some interference by metals which is observed a t platinum (2); however, reduction a t low overpotential perhaps would offer a better approach, in general. The direct reduction of hydrogen peroxide at bare electrodes is not suited for analytical applications. A reverse pulse method in which the hydroxide that is produced upon the reduction of hydrogen peroxide depolarizes mercury permits quantitative determinations based upon the anodic current (3);however, this approach is incompatible with solutions which contain high concentrations of anions that also depolarize mercury. Immobilization of cytochrome c peroxidase (CCP) by adsorption onto pyrolytic graphite yields an electrode a t which a cathodic current that is related to the hydrogen peroxide is produced. Armstrong and Lannon reported that voltammetric peak currents were proportional to hydrogen peroxide concentration up to a t least 70 pM when gentamycin was present as a promoter ( 4 ) , but a cathodic current was not observed in the absence of a promoter. Another limitation was that thermal denaturation of CCP was indicated at 25 "C. Paddock and Bowden (5) have reported direct electron transfer between a CCP-modified electrode and hydrogen peroxide. The half-wave potential at a rotating disk electrode was at 0.7 V vs SHE, a value related to the CCP couple. The system gradually became more irreversible at 4-7 "C; however, some electrocatalytic activity was observed after
2 months in storage. The use of a phosphate buffer rather than N-(2-hydroxyethyl)piperazine-N-2-ethanesulfonic acid and/or the difference in the polishing procedures may account for the disagreement between these reports on CCP. Immobilization of Ru(NH3)2+ into a montmorillonite clay coating on graphite resulted in an electrode that mediated the reduction of hydrogen peroxide at -0.2 V vs SSCE (6). In this mechanistic study, only high concentrations of hydrogen peroxide were used, and the ruthenium complex a t the micromolar level was included in the electrolyte to minimize its loss from the coating. Alkylsilanization of a gold electrode permitted the simultaneous detection of O2 and H202in a mixture because of the suppression of the first cathodic peak in the linear scan voltammetry of the latter species (7). Difficulty in obtaining complete surface coverage of the modifier was suggested, and analytical data were not reported. Updike et al. (8)devised an indirect amperometric sensor for hydrogen peroxide that was based upon the use of an inorganic catalyst for its decomposition to oxygen. A catalyst, such as a sulfide or oxide of ruthenium, was impregnated into the membrane which coats a Clark-type oxygen electrode. The resulting system was suggested to have greater stability than would be realized with an enzyme such as catalase. The present study describes a procedure based upon a modified electrode which has the necessary characteristics for measuring H202with an amperometric flow cell. Because it promotes reduction of H202 at only -0.2 V vs SCE, has long-term stability, and can be used at ambient temperature, this electrode may be advantageous relative to other reported electrodes for this purpose.
EXPERIMENTAL SECTION All chemicalsused were ACS reagent grade, and they were not further purified. The PdC12was obtained from Morton Thiokol, Inc.; NapIrC&,from Strem Chemicals, Inc.; and 30% HzOz,from Mallinckrodt, Inc. Other reagents were obtained from Fisher ScientificCo. The water was distilled in-house and then further purified with a Sybron/Barnstead NANOpure I1 system. The experimentswere performed with BAS 100 electrochemical analyzer (Bioanalytical Systems, Inc.). All potentials were measured and reported vs the Ag/AgCl reference electrode. Glassy carbon electrodes (3.0 mm diameter, Bioanalytical Systems,Inc.) were polished with 1-15 pm (Morton Thiokol, Inc.), 0.3 pm, and 0.1 pm (Fisher Scientific Co.) alumina on a polishing cloth with water as the lubricant. The electrode was rinsed under a stream of water for 5 min before changing the size of the alumina and after the final polishing. Electrochemical pretreatment of the bare electrode before modification did not improve the properties of the modified electrode (9)and also did not have any effect on the hydrogen peroxide reduction reaction on the modified electrode; hence, it was not generally employed. The electrode modification procedure was slightly different from that previously described (9). The plating solution was 0.2 mM NaJrCg, 0.1 mM PdClZ,0.2 M K2S04,and 0.1 M HCl. A polished glassy carbon surface was cycled therein 5 times between 1.2 and -0.3 V at 50 mV 8. This resulted in a film of an iridium oxide that contained some Pd(II), presumably by isomorphic replacement (9). As the plating solution aged, more cycles were
0003-2700/69/0361-2176$01.50/00 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 19, OCTOBER 1, 1989
2177
T
20fi
n
I
to.400
-0.5
$0.0 E
-0.800
+O.Kx)
-0.5
$0 .O
E
(VOLTS)
-0 .800
(VOLTS)
Flgure 1. Effect of scan rate on the voltammetry of 1 mM H,Oz, 0.1 M KCI at the modified electrode. Scan rates, 0.10, 0.16, 0.25, 0.36, 0.49, and 0.64 V s-' for curves with the lowest-to-highest currents,
Linear scan voltammetry of 1.O mM HzOz in the presence and absence of dissolved oxygen at the modified electrode: curve A, air-saturated; curve B, deaerated; scan rate, 0.1 V s-'; electrolyte,0.1
in order.
M KCI.
needed to obtain a given film thickness, as indicated by the peak current for the reduction of the immobilized species. The plating solution was not deaerated. The modified electrode was stable for at least 12 days, whether stored in an electrolyte or used in the experiments. Over two hundred cyclic voltammetry experiments on H202were performed with a single surface without any change in sensitivity; the film was intentionally destroyed after this sequence. Before recording a linear scan voltammogram the modified electrode was cycled between -0.8 and 0.9 V in the test solution;the number of cycles and the cathodic potential excursion were not important, but it was necessary to have the film in its oxidized state before initiating a reduction of the test species. This required a positive potential excursion beyond 0.5 V. Unless otherwise stated, all solutions were deaerated by purging with prepurified nitrogen.
A calibration curve was prepared in 0.1 M KCl at a scan rate of 100 mV s-l. Total faradaic currents were measured at nine points in the range 0.20 to 1.80 mM H202. A linear least-squares fit yielded the following: slope, 28.0 f 0.4 pA L mmol-'; y intercept, 4.3 f 0.5 pA; correlation coefficient, 0.999. The intercept current represents the current due to the immobilized mediator at the peak potential observed for the Hr02reduction. With a film that was ca. 30% thinner (i.e., yielded a y intercept of 2.8 FA rather than 4.3 FA), the slope was essentially the same, 27.8 pA L mmol-'. With KC1 as the supporting electrolyte, the peak potential for the H202 reduction varied from -0.20 to -0.35 V from film to film. A t any given modified electrode, it is constant. As suggested above, the sensitivity for H202was also constant. There are two general limitations of the method at its present stage of development. First, the reduction of dissolved oxygen overlaps that of H202. This is illustrated by Figure 2. A comparison of linear scan voltammograms of oxygencontaining and deaerated H202 solutions shows that the presence of dissolved oxygen increases the current at all potentials that are useful for determining Hz02 The slight shift in the peak potential results from the reduction peak for O2 occurring at a more negative value than that for H202 The currents due to the reductions of oxygen and hydrogen peroxide in Figure 2 are additive; for example, the slopes of calibration curves for hydrogen peroxide in the presence and absence of oxygen are 27.7 f 0.6 and 27.8 f 0.3 pA L mmo1-l) respectively. Because the peak potentials for the curves in Figure 2 are not coincidental, the former data are obtained at -0.24 V, and the latter, at -0.21 V. The correction procedure, subtraction of the contribution of the reduction of oxygen from the total current, requires knowledge of the oxygen concentration, which is readily obtained by an identical experiment but with an unmodified electrode. Use of a flow cell with a dual indicator electrode is the suggested approach. It is noteworthy that the reduction of oxygen at this modified electrode occurs in a single, four-electron step rather than through a pair of two-electronsteps, the mechanism at a bare electrode, because the peroxide which is a product of the first step is electroactive at the modified electrode at potentials that are positive of -0.3 V. The second problem is that, as mentioned in the Experimental Section, in 0.1 M KC1 a preoxidation of the film must be employed in order to activate the modified surface. This would preclude the use of this electrode as a potentiostatic indicator in an amperometric sensor for high-performance liquid chromatography; a pulsed system would be required. However, we have preliminary data which suggest that this problem can be alleviated by use of basic solutions rather than neutral KCl solutions as the supporting electrolyte. In neutral KC1, cyclic voltammetry of the modified electrode results in cathodic and anodic peak potentials of -0.2 and 0.4 V, respectively, whereas at pH 12, the peak potentials are nearly coincidental at -0.3 V.
RESULTS AND DISCUSSION Linear scan voltammetry of the modified electrode from 0.4 to -0.7 V w Ag/AgCl is featureless in deaerated 0.1 M KCl except for a peak at -0.2 V, which is apparently due to an immobilized species. In the presence of H202the current near -0.2 V is increased and the corresponding anodic process at ca. 0.4 V is decreased. At a bare electrode, a current for the reduction of H202is not observed in this range. These results suggest that a mediated reduction occurs at the modified electrode. Figure 1 illustrates linear scan voltammograms for the mediated reduction in 0.1 M KC1. Over the scan rate range of 0.10-1.00 V s-l, the peak current function varied only from 114 to 106 pA s1l2 V-lI2 with a mean and relative standard deviation of 110 f 2 pA s1/2 at n = 8. The same results were obtained with K2S04as the supporting electrolyte. In this series of experiments, the film was only about 40% of the typical thickness so that the contribution of the current due to the reduction of the immobilized mediator was not a significant fraction of the total cathodic current (less than 10% of the peak current in the worst case, 1.00 V s-l). Apparently, the current due to the reduction of H202is limited by diffusion of the analyte in the bulk solution rather than penetration into the film, electron diffusion within the film, or the kinetics of the cross-exchange reaction. Glassy carbon cannot be modified in a plating solution that does not contain Pd(I1); however, platinum can be coated with an iridium oxide film in a mixture that contains 0.2 M K2S04 and 0.2 mM Na21rC16(9). The film is not stable, but the reduction of H20zby linear scan voltammetry at this surface is similar to that at the Pd-containing film. The role of palladium either is limited to stabilizing the film or is duplicated by the presence of the underlayer of platinum. The above data allow two predictions. Because the current is limited by mass transport, a linear calibration curve is expected. Second, as the film apparently is highly conductive and possesses a sufficient concentration of mediator on its surface to obviate the need for substrate penetration, the slope of the working curve should be independent of the thickness of the immobilized layer.
Flgure 2.
Anal. Chem. 1989, 61, 2178-2184
2178
Table I. Linear S c a n V o l t a m m e t r y of E l e c t r o d e in B a s i c S o l u t i o n "
scan rate, \'
s-l
0.01 0.04 0.09 0.16 0.25 0.36 0.49 0.64 0.81 1.00
ipo-112,
HaOIat the M o d i f i e d
V-112 9112
32 31 31 33 33 32 33 33 32 32
E,, V -0.386 -0.419 -0.426 -0.428 -0.432 -0.436 -0.437 -0.439 -0.436 -0.439
aSolution 1.0 mM HzO, in 1.0 M KOH.
The reduction of Hz02 at the modified electrode is dependent upon pH. In an acidic media or in buffered solutions at pH values below 10, measurable currents for the reduction of millimolar levels of H202are not observed at potentials more positive than -0.5 V, the range where the mediated process is expected at the modified electrode. In neutral or basic unbuffered solutions and in buffers of pH 10 or greater, the reduction of H202occurs, and the process is diffusion-controlled, the ideal case for an electroanalytical procedure. For example, the results for the reduction of H20zat the modified
electrode in 1M KOH,which are summarized in Table I, show that the function, iPu-lf2, is independent of scan rate, u, over a wide range. The peak potential does not change markedly with u and is only about 200 mV more negative than that in neutral solution. These observations suggest that KC1 is a suitable supporting electrolyte only because the electrolysis consumes protons thereby causing the interfacial pH to increase. A detailed study of the behavior of this electrode in basic solution is presently under way. LITERATURE CITED Harrar, J. E. Anal. Chem. 1983, 35, 893. Lundback. H.; Johanson, G. Anal. chhn. Acte 1981, 128. 141. Brestovisky, A.; KirowaEisner, E.; Osteryoung, J. Anal. Chem. 1983. 55, 2063. Armstrong, F. A.; Lannon, A. M. J . Am. Chem. Soc. 1987, 109, 7211. Paddock, R. M.; Bowden, E. F. J . .€ktraanal. Chem. 1989, 280, 487. Oyama, N.; Anson, F. C . J . Ekbpanal. Chem. 1988, 199, 467. Fujihira, M.; Muraki, H.; Aoyagui, S. Bull. Chem. Soc. Jpn. 1986, 59. 975. Updike, S. J.; Shults, M. C.; Kosovlch, J. K.; Treichei. I.; Treichei, P. M. Anal. Chem. 1975, 4 7 , 1457. Cox, J. A.; Gadd, S. E.; Das, B. K. J . Electroanal. Chem. 1988, 256, 199.
RE" for review May 1,1989. Accepted July 3,1989. This work was supported by the National Institutes of Health through Grant 1R15GM39948-01.
Polarographic Methods for Ultratrace Cobalt Determination Based on Adsorption-Catalytic Effects in Cobalt(II)-Dioxime-Nitrite Systems' Andrzej Bobrowski Academy of Mining and Metallurgy, Institute of Material Science, Al. Mickiewicza 30, 30-059 Cracow, Poland
Polarogrsphlc curves obtained in the new systems Co( II)dioxfme/nl~xbne,cu-bensn dioxime, dhnethylglyoxkne ( D ~ o ) , a-fwli dioxkne/NaNO, are shown to be of an adeorptlonc a w netwe. The direct cucfent and dwlerentlai pulse peak currents of cobaH increase in the Co(II)-dioxime-NaNO, systems by as much as 3 to 4 orders of magnitude, whlch enables the determlnation of uttratraces of Co wlth hlgh senshRy,-and-. Thecatalytkeffectdoes not appear in a solution containing the NI( II)-dioxime complexes and NaNO,. The hlghest senstthrlty of the determlnatlon is obtalned In the system Co( II)-nloxime-NaNO,. The reductlon of the Co( I I)-nloxlme complex In the presence of M e ions has been investigated and the effects of pH, buffer capaclty, lonlc strength, electrolyte composltlon, and Instrumental parameters have been studied. The opthum condltlons for the determhatkn of cobatl In the pmence of a great excess of NI and Zn have been established. The determlnatkn Umlt, restrkbd by the amount of Co h the blank test, was 3 X lo-'' M Co. I n the method a conslderably smaller influence of NI and Zn on Co determinatlon was observed compared wHh the well-known adsorption voltammetrk method of Co determinatlon In the form of a Co-DMG complex. Dedicated to the memory of Professor W. Kemula. 0003-2700/89/0361-2178$01.50/0
During the last decade the adsorption-voltammetric method (AV) based on the adsorption accumulation of the dimethylglyoximate of Ni or Co on stationary electrodes such as a hanging mercury drop electrode (HMDE)/(l-IO) mercury f i i electrode (11,12),or a chemically modified electrode (13) have found wide application in trace analysis. Opposite opinions exist concerning the mechanism of the electrode process involving the mentioned complexes. Some investigators (14-20) maintain that the reduction process of the complexes is accompanied by a catalytic evolution of hydrogen; others believe that the increase of the current observed for Ni and Co in the presence of dimethylglyoxime (DMG) is due exclusively to the adsorptive accumulation of the complexes on the electrode surface (1,8, 12, 21, 22). Although the AV method for Ni in the presence of DMG proved to be the most sensitive and suitable analytical technique, a great excess of zinc as well as nickel makes Co determination impossible (4, 10, 14, 23, 24). This limitation prompted the development of a still more sensitive and selective method for Co determination. The aim of earlier investigations carried out by the author (25,using s) modern polarographic techniques (differential pulse, normal pulse, linear potential fast sweep) was to determine the polarographic properties of the Co-DMG complex as well as Co complexes with other dioximes such as 1,2-~yclohexanedionedioxime @ 1989 American Chemical Society
